This invention relates to implantable stimulators generally and more particularly to implantable cardioverters and defibrillators.
Cardioversion and defibrillation pulses have traditionally been synchronized to detected cardiac depolarizations. In the context of both external and implantable cardioverters and/or defibrillators, synchronization has been accomplished by means of an R-wave detector, which triggers delivery of a cardioversion or defibrillation pulse a short interval thereafter.
The interval between R-wave detection and delivery of a cardioversion or defibrillation pulse has varied somewhat in different prior art implementations. Most cases the delay appears to be an inherent function of the circuitry of the device, rather than a result of any attempt to produce ideal timing of the defibrillation pulse with respect to the detected R-wave. However, U.S. Pat. No. 4,830,006 issued to Haluska et al. suggests that the delay between R-wave detection from the intracardiac EGM to the delivery of the cardioversion shock should be adjustable by the physician to achieve optimal synchronization.
Traditional R-wave detectors have comprised a bandpass filter and a threshold detector and have generally been used for synchronization purposes. However, synchronization to other features of the electrogram has been proposed. For example, U.S. Pat. No. 4,559,946 issued to Mower suggests synchronization to the point of peak slope of the intracardiac EGM.
As a practical matter, intracardiac or surface R-wave detection circuitry typically detects the occurrence of an R-wave at a point in time which typically occurs somewhat after the onset of the R-wave, and which varies depending upon the morphology of the particular R-wave being sensed. As a result, the synchronization of the cardioversion pulse to the R-wave is somewhat variable. In the context of implantable devices, the R-wave detectors used for synchronization has been coupled to an electrode pair on or in the heart or to an electrode pair comprising a first electrode in or on the heart and a second, remote electrode.
An earlier attempt to provide improved control of the relationship between the depolarization of the heart and the delivery of a cardioversion pulse is set forth in U.S. patent application Ser. No. 07/630,698, for a Paced Cardioversion, filed Dec. 20, 1990, by Mehra, incorporated herein by reference in its entirety. In this application, the inventor proposed overdrive pacing in response to detection of the tachyarrhythmia, and synchronization of the delivered cardioversion pulse to the overdrive pacing pulse. While it is believed that this approach is workable, it does require additional expenditure of energy in the form of overdrive pacing pulses.